Optical encoders resolve the position and movement of a rotating shaft as described in U.S. Pat. Nos. 4,266,125 and 4,259,570, incorporated herein by reference. These prior-art optical encoders modulate a light beam with a code wheel that is mounted to the shaft and with a stationary phase plate. The modulated light beams are converted into electrical signals by light detectors. From these electrical signals, the speed and direction of the shaft rotation can be ascertained.
Prior-art optical encoders have numerous parts as shown in FIG. 1. These include light emitting diodes (LEDs), lenses for collimating the light beams, a code wheel mounted on the shaft for modulating the light beams, a phase plate for splitting the light beams and for introducing a phase difference of 90.degree. between them, elaborate lenses, and light detectors for receiving the modulated light and converting it into electrical signals. Some of these parts are cumbersome to manufacture such as the bifurcated, truncated lenses for light detectors. The prior-art encoders have multiple sources of errors. The light beams emitted from the multiple LEDs may not be identical and the code wheel and the phase plate may not be aligned. Also, the distance between light detectors renders them sensitive to gradients in the light beams.
In accordance with the illustrated preferred embodiment of the present invention, an optical encoder has fewer parts, is therefore simpler and cheaper to manufacture, and has greater accuracy than prior-art encoders. The optical encoder has one light source producing one light beam, a simple lens for collimating the light beam, a code wheel for modulating the light beam, and light detectors for receiving the light beam and including the function of the phase plate. By reducing the number of parts and making them easier to fabricate, the manufacturing procedure is simpler and cheaper. Greater accuracy is achieved by using a single light source instead of multiple light sources. Also, the light detectors are placed immediately adjacent to one another, and each light detector receives information belonging to two channels. Furthermore, the light detector outputs are connected in a four-way push-pull configuration which cancels errors due to gradients in the light beam and misalignment of the code wheel.